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Circulatory System

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Circulatory System - Lesson Summary

The circulatory system is of two types – open and closed. In open circulatory system, the blood pumped from the heart reaches the body cavities through large blood vessels. In closed circulatory system, the blood pumped from the heart passes through closed blood vessels to the tissues.
The heart has many chambers, which vary in different animals. Fishes have a two-chambered heart, all amphibians and reptiles with the exception of crocodiles have a three-chambered heart, all crocodiles, birds and mammals have four-chambered hearts.
In fishes, blood passes through the heart only once during circulation and hence called single circulation. In frogs and reptiles, blood passes through the heart twice and gets mixed in the single ventricle during circulation and hence this is called incomplete double circulation. In birds and mammals, blood passes through the heart twice during circulation and hence this is called double circulation.

During blood circulation, arteries carry oxygenated blood from the heart and branch out to form small arterioles that end in capillaries to reach the tissues. These capillaries collect deoxygenated blood from tissues and converge to form venules, which finally converge to form veins.

However, there are two exceptions in which the artery and vein perform different functions – the pulmonary artery and the pulmonary vein.

The human heart is about the size of a fist and lies in the thorax between the two lungs, slightly tilted towards the left lung. It is protected by the pericardium, which encloses the pericardial fluid.
The inter-atrial septum separates the two atria, while the inter-ventricular septum separates the two ventricles and the atrio-ventricular septum separates the atria from their respective ventricles.

The openings between the chambers of the heart are guarded by three types of valves that allow the flow of blood in one direction. With the closure of the tricuspid and bicuspid valves, the first sound, ‘lub’ is produced and the closure of the semilunar valves produces the second sound, ‘dub’.
Specific regions in the heart consist of specialised cardiac muscles called nodal tissues. The nodal tissues include Sino-atrial node (SAN) and Atrio-ventricular node (AVN). AV bundle from the AVN branches into the right and left bundles in the ventricles to form minute Purkinje fibres. All these fibres together form the Bundle of His.

Here, the SAN can automatically generate around 70 to 75 action potentials or electrical impulses per minute. This action potential is transmitted to every other part of the heart through the AV node and Bundle of His to bring about the rhythmic contraction of the heart. Hence, the SAN is called the pacemaker of the heart. 


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